A compression ratio of an internal combustion engine is defined as a ratio of a cylinder volume when a piston within the cylinder is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine. In conventional engines, the compression ratio is fixed. However, in variable compression ratio (VCR) engines, the engine may be equipped with various mechanisms to alter (e.g., mechanically) the volumetric ratio between the piston TDC and BDC, allowing the compression ratio to be varied as engine operating conditions change. As a non-limiting example, the VCR engine may be configured with a mechanical piston displacement changing mechanism (e.g., an eccentric) that moves the piston closer to or further from the cylinder head, thereby changing the size of the combustion chambers. Still other engines may mechanically alter a cylinder head volume.
One example approach for leveraging the benefits of a VCR mechanism is shown by Kolmanovsky et al in U.S. Pat. No. 6,553,949. Therein, a higher compression ratio may be used in low engine speed-load regions to increase thermal efficiency. A lower compression ratio may be used in higher engine speed-load regions. Further, knock may be mitigated in the low speed-load region using spark retard, while knock is mitigated in the higher speed-load region via adjustments to the compression ratio. When the engine becomes spark limited while operating in the higher compression ratio, the combustion phasing retard may erode the thermal efficiency benefit of the compression ratio. At that time, the compression ratio may be lowered while spark timing is advanced so as to provide a more efficient balance between combustion phasing and thermal efficiency.
However, the inventors herein have recognized potential issues with such systems. As one example, if an engine shutdown is initiated while the engine is operating at a higher compression ratio, there may be significant engine shake and vibrations due to increased piston bounce-back from the higher pressure in the higher compression ratio cylinder. The increased vibrations may disturb vehicle occupants and cause objectionable noise. The issue may be exacerbated in certain engine speed regions traversed during an engine shutdown when operating in the higher compression ratio.
In one example, the issues described above may be addressed by a method, comprising: operating an engine at a compression ratio via mechanical adjustments to a variable compression ratio mechanism, the compression ratio selected based on operating conditions; and during an engine shutdown, lowering the compression ratio after a last combustion event and before disabling fuel and spark. In this way, engine shake during shutdown may be reduced, minimizing disturbances to vehicle occupants.
As one example, the compression ratio is lowered during engine shutdown responsive to predicted or detected engine shake during the engine shutdown, the engine shake predicted based on engine speed when the engine shutdown is initiated and detected based on output from a vibration sensor coupled to an engine block. In other examples, such as when engine shake is not predicted, the compression ratio may be maintained during engine shutdown, such as when the shutdown is initiated by an engine controller responsive to idle-stop conditions being met. Engine shake may be predicted based on the engine speed being above a threshold engine speed and/or the compression ratio being above a threshold compression ratio, for example. By reducing the compression ratio when engine shake is predicted, piston bounce-back may be decreased during engine shutdown, thereby reducing engine shake and vibrations and the resulting disturbance to vehicle occupants. Further, by maintaining the compression ratio when engine shake is not predicted, engine efficiency may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.